One of the units of measurement of induced polarization in the time domain. True chargeability is the ratio of the over- or secondary voltage, V_s, to the observed voltage, V₀, applied by way of an electrode array so that M = V_s/V₀, expressed as a percentage or as millivolts per volt; this quantity is independent of topographic effects and of electrode geometry and is thus a good measure of induced polarization. In reality, what is measured is the apparent chargeability (M_a) which is the area (A) beneath the voltage-time decay curve over a defined time interval (t₁ to t₂) and normalized by the supposed steady-state primary voltage, V_p, such that M_a=A/V_p×∫12₁₁ of V(t)dt, in units of mVs/V.